Research On Key Technologies About HF Modem

The key technologies about HF modem are studied in this thesis.They mainly include synchronous acquisition technology,frequency offset estimation technology,channel sparsification technology and Turbo equalization technology.Firstly,two synchronous acquisition algorithms based on sliding group matched filter and sliding correlation group FFT are studied.The performance of the two algorithms are simulated and compared in different frequency offset.The simulation results show that the latter is more suitable for HF communication system.Secondly,three frequency offset estimation algorithms of interpolation,quadratic interpolation and frequency domain correlation are elaborated in detail.The mean estimated error performance of the three algorithms is simulated and analyzed from two dimensions of SNR and frequency offset.Simulation shows that the mean estimated error fluctuation of the three algorithms with various SNR is small when the frequency offset is fixed.But when the SNR is fixed,the mean estimated error fluctuation of interpolation algorithm and quadratic interpolation algorithm is large while the frequency domain correlation algorithm is small with various frequency offset.Therefore,the estimation performance of frequency domain correlation algorithm is better.Thirdly,a HF channel sparsification method is proposed and simulated.The simulation results show that the proposed algorithm improves the bit error rate performance of HF communication systems under multipath channel compared with HF communication systems without this algorithm.Therefore,with channel sparsification algorithm,the reliability of HF communication systems can be improved.Finally,Turbo equalization techniques of LDPC,convolutional and double-binary Turbo codes are studied.Firstly,the principles of MMSE(Minimum Mean Square Error)equalization,SIC(Soft Interference Cancellation)equalization and the hybrid MMSE+SIC equalization algorithms are analyzed.Secondly,the coding and decoding principles of the three codes are described in detail.Finally,the performance of the algorithms are simulated.Simulation shows that the performance of LDPC and double-binary Turbo codes are superior to convolutional codes with short transmission frames under the AWGN(Additional White Gaussian Noise)channel,as well as the ISI(Inter Symbol Interference)channel without Turbo equalization iterations.However,for an ISI channel with Turbo equalization,the performance of double-binary Turbo codes and the LDPC codes are poorer than that of convolutional codes with short transmission frames at the same number of iterations.In addition,the decoding performance of LDPC and convolutional codes are analyzed with EXIT(Extrinsic Information Transfer)chart method.